A compound semiconductor provides an electron mobility which is considerably faster than that in silicon, and therefore is good at high-speed signal processing and has the property of operating at a low voltage, reacting to light, or emitting a microwave. From such outstanding physical properties, a device using the compound semiconductor is expected to exceed the physical-properties range of a semiconductor silicon device which is currently dominant.
Until now, for example, a Schottky barrier diode etc. is known in which a nitride-type compound-semiconductor single crystal layer is stacked on a Si substrate via a buffer layer (see Japanese Laid-open Patent No. 2003-60212).
However, in the conventional device manufactured by using the compound semiconductors mentioned above, a buffer layer comprising a compound semiconductor single crystal has a high resistance, and causes energy loss. Further, electric-field concentration occurs in an electrode, and a breakdown voltage becomes quite low compared with an ideal status, thus being hard to say that it is sufficiently practical.
Especially it is difficult for a diode which has the basic structure to be sufficiently practical, because of the energy loss due to much leakage current.
In such a conventional one as mentioned above, the high resistance is caused by a low carrier concentration of a compound semiconductor single crystal buffer layer of a nitride represented by hexagonal GaN, hexagonal AlN, etc. It is, however, physically difficult to increase the carrier concentration in such a buffer layer.
Further, the improvement in reducing the electric-field concentration in the electrode is not sufficiently achieved, either.